Eldritch Location: Besides being a sapient, talking geological formation the place can contain multiple separate biomes with locations from hundreds of years apart underground, going down it all the way somehow sends you back to the top, and if you keep what you came for, the ladder to the surface is implied to be endless.
Interactive Narrator: Played with; while he functions very much as one, it's made explicit from the very beginning that he is an actual character in the game world, he just happens to be a faceless seemingly-omniscient being with an omnipresent voice like most narrators.
Jerk with a Heart of Gold: He has a rather cruel sense of humor and oftentimes makes jokes that are in bad taste, but he seems to have the players interest at heart. He encourages them to look past the failures and see what lessons there are to be learned, and maintains faith that when the player makes their own journey into The Cave, they'll be ready to make the right decisions.
Journey to the Center of the Mind: The game is this for both the cave and the characters. It seems to be very much telepathic, as it presents precisely those defining circumstances to those characters who choose to enter it.
Character Development: Either the character development happened during the journey, or the journey merely reviews character development that already occurred. The bad endings depict them having succumbed to inner demons in the past, and either dealing with or choosing to ignore the fallout of their decisions. The Good Endings show what is either them biting down on their inherent flaws at the last minute, or their expressing sorrow over not doing so (it's ambiguous which actually happened). Either way, they leave The Cave with an appreciation of moral consequences.
Double Meaning: All of The Cave's introductions for the characters become a lot more sinister once you see what they are entering The Cave for.
Heroic Mime: The flashbacks make it clear they're capable of talking, but in-game they don't say anything.
Anti-Villain: In the Good Endings, if you interpret it as the characters having made the bad decisions in the past and regretted them after the journey into The Cave.
Anti-Hero: Also in the Good Endings, if you instead interpret it as the characters having refused to make the evil decisions in the past, despite all the villainous build-up, backstory, and inner demons, and the journey into The Cave was just a review of that history.
"He is on a quest for a sword of unequaled power and prestige...but will he find it before anyone else gets hurt?"
A man in full plate armor, whose helmet completely covers his face. Can create a barrier around him that is immune to all damage, but he cannot move while it is active.
Epic Fail: In his bad ending, as a result of trying to take down the dragon, the entire kingdom goes up in flames.
Excalibur in the Stone: His object of desire. He can't pull it from the stone until his partners use some dynamite to loosen it.
Legendary Impostor: He's actually a peasant who stole the armor from the real (late) knight.
Multiple Endings: If the knight chooses to take the sword from the cave, his ending is to burn to death along with the king and the country when the dragon attacks. If he leaves what he desires most behind, then he reveals who he really is to the king, who subsequently prepares his soldiers for battle instead of depending on an irresponsible boy to save him.
Freudian Excuse: He's clearly shown to have suffered from Parental Neglect. It's also why he's so sure a pink teddy bear will conquer his "true love": he had the same as a child and it was his only friend.
"She is here to right a wrong a million years in the making. Fortunately for her, yesterday is a new day."
From The Future comes this neon-decorated young woman. She comes determined to fix the past to save her future equipped with the ability to phase through small barriers.
Disproportionate Retribution: She killed the progenitor of a family dating back millions of years simply because her current generation's member won Greatest Employee of All Time at the Mega Corp. where they were employed, and he may have lorded it over her.
Science Is Useless: Man, those hover-boots sure are flashy. Too bad they don't let her do more than float a couple inches off the ground and still subject her to every other gravity-based issue the other characters can have.
"She is on the cusp of a great discovery for all of humankind...and a hundred million lives hang in the balance."
Plump, female, highly intelligent, very morally questionable aims. Can use SCIENCE! to subvert various control panels and other devices.
Greed: For no stated reason other than cold hard cash, she assisted in the launch of a nuclear missile which killed over a hundred million people.
I Just Want To Be Free: With the darkest possible tone. When they felt too oppressed by their proper, upstanding (but very loving and reasonable) parents, they simply killed them with rat poison.
Kick the Dog: Their parents bought them each ponies, which both died in a "mysterious" barn fire.
Multiple Endings: If the twins leave the cave with the box of rat poison, they die along with their parents. If they leave the box behind, they realize the importance of family at the last second and make up with their parents after warning them not to eat anything.
Spoiled Brat: Listening to Daddy may cause him to say that he will each buy them another pony if they're good.